SQL-DATABASE ERROR

Database error in WoltLab Burning Board: Link-ID == false, connect failed
mysql error: Can't connect to MySQL server on 'mysql.service' (113)
mysql error number: 2003
Date: 17.12.2018 @ 14:36
Script: /wbblite/board.php?boardid=22&sid=2149159f5630ee295eb2b2ab74b141a2
Referer: